Power line conditioning systems for monitoring the quality of an AC line voltage and restoring an unsatisfactory line voltage to within a specified voltage range are known. There are various causes for a line voltage to be outside of its specified range. For example, a line-to-ground fault on one phase can cause the line-to-line voltages to become unequal.
Another power line quality problem is the presence of harmonic disturbances on the line, as can be caused by non-linear loads. Such harmonic disturbances take the form of AC currents on the power line at frequencies other than the nominal line frequency of 60 Hz.
In one type of conventional power line conditioning system, sometimes referred to as an active VAR (volt-ampere reactive) generator, reactive energy is injected into or withdrawn from the line to restore the line voltage to the desired level. Specifically, if it is determined that the line voltage is too high, then an inductive current is injected into the line to lower the line voltage; whereas, if the line voltage is too low, then a capacitive current is injected to raise the line voltage. Active VAR generators include an inverter connected in series between the transmission line and a DC energy storage device, such as a battery, DC bus capacitor, or other energy storage device used to provide power to the inverter. The inverter is controlled to convert the DC energy stored in the DC energy storage device into a current waveform which is injected into or withdrawn from the transmission line to restore the line voltage to the desired condition.
Various control techniques are possible for an active VAR generator in order to determine the appropriate amount of reactive energy to inject into or withdraw from the line. In accordance with one such technique, voltage amplitudes are sampled on a phase-by-phase basis and used to determine the necessary current to restore the voltage to the desired level. However, such a phase-by-phase analysis of the line voltage may result in an injected current demand that exceeds the system capacity or is ineffective in restoring voltage balance, since this type of analysis ignores interaction between the phases.
Another control technique for determining the current levels necessary to restore the line voltage to the desired level includes analyzing the sequence components of the line voltage and is sometimes referred to as phase sequence separation or simply, sequence separation. Unbalanced voltage lines can be represented as the sum of a forward and backward rotating vector of fixed magnitude and frequency. The forward rotating vector is referred to as the positive sequence component and the backward rotating vector is referred to as the negative sequence component. In order to balance an unbalanced line condition, it is desirable to bring the negative sequence component to zero and to bring the positive sequence component to within some small percentage of a predetermined value.
One type of sequence separation utilizes synchronous reference frame control in which measured three-phase voltages or currents are transformed into synchronously rotating D axis and Q axis reference frame quantities. The D-Q reference frame quantities are used to generate inverter control signals that are transformed back to three-phase static reference frame quantities for use by the inverter. For example, in an active VAR generator utilizing synchronous reference frame control, each phase of a three-phase line is sampled and the three-phase quantities are transformed into a stationary two phase X-Y axis reference frame quantities. The stationary X-Y reference frame quantities are transformed into synchronously rotating two-phase quantities and proportional-integral (PI) control is used to determine the currents to be injected into, or withdrawn from the line. The rotating D-Q currents are transformed back to a static three-phase reference frame to generate the control signals for the inverter of the active VAR generator. A phase locked loop (PLL) is used to lock the rotating reference frame to the monitored line, so that current is injected into the line or withdrawn from the line with the correct phase. Low pass filters are used to filter the second harmonic component attributable to the sequence component of opposite polarity. However, a tradeoff exists between the speed of response and elimination of the second harmonic components, since low pass filters result in significant response delays.
Another type of power system that utilizes synchronous reference frame sequence separation is an active rectifier. Active rectifiers convert AC power into DC power to generate a regulated DC bus voltage. Non-linear loads contribute to the generation of undesirable harmonics on the DC bus. In particular, the negative sequence component of the AC line voltage produces second harmonic, 120 Hz ripple on the DC bus. Further, 5th, 7th, 11th and 13th line voltage harmonics produce 6th and 12th harmonics on the DC bus. A PLL is used to synchronize the active rectifier to the AC line and a low pass filter is used to smooth the estimated frequency signal. Here again, there is a tradeoff between the speed of response and the elimination of harmonic components.